Finally tonight, here, the making of a biblical blockbuster, "Noah," one of the best-known stories of the bible, brought to life on a massive scale. And the biggest challenge for the filmmakers, the ark. "Nightline" anchor, Dan Harris, behind the scenes, tonight. A great flood is coming. We build a vessel to survive the storm. We build an ark. Reporter: Early critics may have taken swipes at "Noah" for veering too far from the biblical version. But you cannot accuse the filmmakers of not having gone the extra mile to create a realistic ark. Action. Reporter: Director Darren aronofsky using the book of genesis as a blueprint. It basically describes a box. Reporter: Building this mammoth ark in oyster bay, New York. Exterior is 85 feet wide. Reporter: It took this man, production designer mark Friedberg, nearly six months to build, using materials actually available to Noah and his family, like twigs, straw, even volcanic rock. It was built to float. Reporter: It's not pretty. But it is tough. It even withstood superstorm sandy, which slammed the shores of Long Island, during filming. All that creeps. All that crawls. All that slithers. Reporter: We remember how the story goes, two of every creature. The ramp was there to serve one function. For reptiles to get up there. For mammals to get up there. Reporter: Impressive, even for Noah himself. The first time I came out for the exterior ark stuff, it was an experience. Reporter: After pouring $125 million into this movie, the studio is now hoping it makes waves at the box office. Dan Harris, ABC news, New York.

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